Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Purchase and retirement of common stock, shares	33,953,127
Purchase and retirement of common stock, shares	33,900,000	119,823,621	85,141,691
Dividends declared on common stock per share (in dollars per share)	$ 0.40	$ 0.40	$ 0.40
Issuance of shares of common stock pursuant to employee stock plans	2,166,861	2,506,067	12,289,537